@CappuccinoKid 

Thank you for your feedback.

2.) apps not being used that are transmitting data or allowing location access 

 

This one might be relevant. Take Gmail for example, you might not open it but it will keep receiving emails from others.

So some system applications run in the background, which would cost an amount of online time.

  @CappuccinoKid 

Yeah that is a common concern.  What is happening is that is likely background actions.  Anything that would use data would, even background actions like checking or doing app updates, background app refreshes, or location checks counts toward the data usage.  Basically anything that requires connection to the internet will count as data usage.  Unfortunately there is no way for the router to determine what type of tarffic is limited.  Access to the internet is access to the interent and that is what it is looking at.  What you might want to try is the built in feature called Screen Time.  It is found in the iPhone or iPad settings app.  I can confirm this feature is available from at least iOS 12 and up.